A plotting compass is placed near a long, straight wire carrying a current. Explain what happens to the plotting compass when the current in the wire is switched on, and what would happen to the compass reading if the current in the wire were then doubled.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student investigates the effect of a current-carrying wire on a nearby plotting compass. The compass is placed a fixed distance from the wire. Initially there is no current flowing through the wire.
Model answer (4 marks)
When the current is switched on the wire produces a magnetic field that circles the wire.
The plotting compass, whose needle is free to rotate, aligns itself with this new magnetic field and therefore deflects from its original position.
If the current is doubled the magnetic field around the wire becomes twice as strong.
Consequently the compass needle deflects further from its original position – the compass reading increases.
The plotting compass, whose needle is free to rotate, aligns itself with this new magnetic field and therefore deflects from its original position.
If the current is doubled the magnetic field around the wire becomes twice as strong.
Consequently the compass needle deflects further from its original position – the compass reading increases.
Examiner tips
- Use the word "deflect" and mention the needle aligns with the field. Show the relationship B∝I to justify the larger deflection when current doubles. Keep the answer short – 4 marks only.
- common_mistakes
- :
- Saying the compass turns to point north instead of aligning with the wire’s field. Forgetting to mention that the deflection increases when the current is doubled.
Mark scheme (4 marks)
- When the current is switched on, the wire produces a magnetic field around it
- The plotting compass deflects / changes direction / aligns with the magnetic field produced by the wire
- When the current is doubled, the magnetic field around the wire becomes stronger
- The compass reading/deflection is greater / the compass needle deflects further from its original position (because the field is stronger)
Key terms in this question
Related
- All Edexcel GCSE Physics (1PH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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